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GENEVA — Operators of the world's largest atom smasher restarted their massive machine Sunday in a run up to experiments probing secrets of the universe, a spokeswoman said.

The European Organization for Nuclear Research, or CERN, sent low energy beams of protons in both directions around the 17-mile tunnel housing the Large Hadron Collider under the Swiss-French border at Geneva, said Christine Sutton.

After a cautious trial period, CERN plans to ramp up the energy of the beams to unprecedented levels and start record-setting collisions of protons by late March, Sutton said.

The restart follows a 2 1/2 month winter shutdown during which scientists made improvements and checked out the smasher's ability to collide protons at energies three times greater than has ever been achieved previously.

The new collisions are expected to shatter the subatomic particles and reveal still smaller fragments and forces than previously achieved on any collider, including the previous record-holder — the Tevatron at Fermilab outside Chicago.

The Large Hadron Collider was built to examine suspected phenomena such as dark matter, antimatter and ultimately the creation of the universe billions of years ago, which many theorize occurred as an explosion known as the Big Bang.

The restart follows successful trial runs late last year when CERN showed that it had made a big comeback from its initial Sept. 10, 2008, startup with great fanfare. The machine was sidetracked nine days later when a badly soldered electrical splice overheated and set off a chain of damage to the magnets and other parts of the collider.

CERN had to undertake a $40 million program of repairs and improvements over 14 months before it was ready to retry the machine at the end of November. Then the the collider performed almost flawlessly, giving scientists valuable data in the four-week run before Christmas.

"They learned a lot which they've gone away and digested, and now they're trying to make adjustments," Sutton said.

CERN specialists have checked out and improved electrical connections and other parts of the machine since the shutdown, but still want to take further steps to make sure the Large Hadron Collider is ready to operate at higher energy.

"There's a long way to go between getting the first bunches of protons to go around and actually getting the machine to its top working levels," Sutton said. "It's a lot like having designed a Formula One racing car. The first time you send it out, the guy doesn't go round the circuit as fast as he can. You have to learn about the controls, how the car handles."

At its greatest energy, the atom smasher collided two beams of circulating particles traveling in opposite directions at 1.18 trillion electron volts, or TeV, about 20 percent higher than the previous record set at Fermilab.

After the current cautious restart, CERN will ramp up the energy pushing the beams of protons still higher, to 3 1/2 times the highest levels reached in Chicago. The showers of particles created at that level are expected to reveal still more about the makeup of matter.

The long-term goal, after more modifications, will be to run the proton beams at 7 TeV in each direction, but CERN has decided that it will continue its cautious approach and run at 3.5 TeV for 18-24 months. Then a long shutdown will allow for further improvements for operation at the full design energy.

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